Glass substrate for information recording medium and method for producing the same

ABSTRACT

There is provided a glass substrate which has the properties required in the use as a substrate for an information recording medium of the next generation such as a perpendicular magnetic recording system, and can be applied as a substrate for an information recording medium of the next generation particularly on the premise of using the glass substrate in a dynamic environment. More particularly, there is provided a glass substrate for an information recording medium which has sufficiently high surface hardness, has a good balance between specific gravity and mechanical strength, and has high strength to withstand high speed rotation or drop impact, and which can be produced with a high productivity adequate for a direct press method, without the occurrence of bubbles in the glass blank or reboil upon pressing even if arsenic components or antimony components are not substantially used. A glass substrate for an information recording medium, includes, as expressed in terms of percent by mass on the oxide basis: 52 to 67% of SiO 2 , 3 to 15% of Al 2 O 3 , and 0.2 to 8% of P 2 O 5 , the glass substrate having a Young&#39;s modulus of 85 GPa or greater, a specific gravity of 2.60 or less, and a ratio of Young&#39;s modulus to specific gravity (Young&#39;s modulus/specific gravity) of 33.0 or greater.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2009-041515, filed on Feb. 24,2009, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a glass substrate for magneticinformation recording medium. More particularly, the present inventionprovides a glass substrate having the properties required of futureinformation recording medium substrate, which glass substrate has lowspecific gravity, high Young's modulus and excellent Vickers hardness inresponse to the future densification of magnetic information recordingmedia, as well as has a highly smooth surface roughness after processingand excellent head sliding properties.

The term “information recording medium” according to the presentinvention means a magnetic information recording medium that can be usedin fixed type hard disks, removable hard disks or card type hard disks,which are used as the hard disks for personal computers; hard disks fordigital video cameras, digital cameras or audio systems; hard disks forautomobile navigators; hard disks for mobile telephones; or hard disksfor various electronic devices.

2. Description of the Related Art

In order to cope with recent development of personal computers formulti-media purposes and to cope with handling of large amounts of datasuch as video files and audio files by digital video cameras, digitalcameras and portable audio devices, there is a demand for magneticinformation recording devices with larger capacities. As a result, it isincreasingly required of magnetic information recording media to acquirehigher recording density year after year.

In response to this, adoption and mass production of perpendicularmagnetic recording system are in progress. This perpendicular magneticrecording system is required to have more excellent heat resistance ofsubstrates and smoothness of surfaces than conventional systems.Furthermore, conventionally, most of magnetic information recordingapparatuses are used in a static environment, such as in the use inpersonal computers; however, in recent years, more of the recordingapparatuses are put to use in a dynamic environment, which isrepresented by portable audio devices and the like. For this reason, itis becoming increasingly important that substrates for informationrecording media of next generation have lower specific gravity so as toreduce the burden on spindle motors, and higher mechanical strength forpreventing hard disk crash. However, having high mechanical strength andexcellent mass producibility is in a contradictory relationship withhaving low specific gravity, and thus research is being conducted on howthese factors can be put in balance in the substrates for informationrecording media of next generation, for which the use in a dynamicenvironment is taken into consideration.

Materials that are used in the substrates for information recordingmedia include aluminum (Al) alloys, glass, crystallized glass and thelike, and glass and crystallized glass have an advantage over Al alloysin view of having high Vickers hardness and the like. Furthermore,crystallized glass generally has an advantage over glass in view ofhaving higher Young's modulus. However, in recent years, there is ademand for a higher level of post-processing surface smoothness, but itis becoming more difficult to obtain the required surface smoothness inthe crystallized glass. On the other hand, conventional glass substratesare not sufficiently satisfactory in terms of the Young's modulus thatis required of the substrates for information recording media of nextgeneration, and even if those currently established processingtechnologies are applied, desired surface properties may not beobtained.

In the case of using glass materials, direct press method is utilized bywhich molten glass is directly pressed, in order to produce disk-shapedsubstrates having a thickness of 1 mm or less at low cost.

In the direct press method, arsenic or antimony components have beenused as refining agents so as to remove bubbles from molten glass whenglass is melted. In recent year, however, there are rising concernsabout the adverse effects of the refining agents on human body andenvironment, and thus it is requested to reduce the content, or not touse the refining agents at all.

Japanese Patent Application Laid-Open (JP-A) No. 2005-302289 discloses asubstrate for an information recording medium made of glass, but thesubstrate does not have sufficient Vickers hardness and does not havesufficiently satisfactory properties that are required for the use as asubstrate for an information recording medium of the next generation.The substrate also tends to have a low ratio of Young's modulus to thespecific gravity.

JP-A No. 2001-19467 proposes a crystallized glass for magnetic disks,but this crystallized glass exhibits a difference in processing or adifference in the etching rate between the precipitated crystals and theglass blank, so that the crystallized glass cannot sufficiently satisfythe surface properties of the currently required level of Ra<2 A(angstrom).

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a glass substratewhich has the properties required for the use as a substrate for aninformation recording medium of the next generation, which isrepresented by a perpendicular magnetic recording system or the like,and can be applied as a substrate for an information recording medium ofthe next generation particularly on the premise of using the glasssubstrate in a dynamic environment. More particularly, it is an objectof the present invention to provide a glass substrate for an informationrecording medium, which has sufficiently high surface hardness, has agood balance between specific gravity and mechanical strength, and hashigh strength to withstand high speed rotation or drop impact, and whichcan be produced with a high productivity adequate for a direct pressmethod, without the occurrence of bubbles in the glass blank or reboilupon pressing even if arsenic components or antimony components that arefeared to cause adverse effects on human body and environment, are notsubstantially used.

The present inventors devotedly repeated experiments and studies toachieve the objects described above, and as a result, they found thatwhen a glass substrate is produced by setting the content ranges ofspecific components constituting a glass, the specific gravity andYoung's modulus of the glass, and the ratio of Young's modulus tospecific gravity (this is a value determined by Young's modulus/specificgravity, and represents the difficulty of deformation of the material),to values in specific ranges, the glass substrate can be applied as asubstrate for an information recording medium of the next generation. Inaddition, the inventors also found that the glass substrate has a highlysmooth substrate surface which makes it possible to cope with a decreasein flying head of the magnetic head, and is also excellent in the dropstrength (>1200 G) at the time of mounting in a drive. Morespecifically, the present invention provides the following.

(Constitution 1)

A glass substrate for an information recording medium, including, asexpressed in terms of percent by mass on the oxide basis:

52 to 67% of SiO₂,

3 to 15% of Al₂O₃, and

0.2 to 8% of P₂O₅,

the glass substrate having a Young's modulus of 85 GPa or greater, aspecific gravity of 2.60 or less, and a ratio of Young's modulus tospecific gravity (Young's modulus/specific gravity) of 33.0 or greater.

(Constitution 2)

The glass substrate for the information recording medium according toconstitution 1, including, as expressed in terms of percent by mass onthe oxide basis:

2.9% to 8% of Li₂O,

wherein the total amount of MgO, CaO, ZnO, ZrO₂ and TiO₂ is 5 to 20%,and

the total amount of CeO₂ and/or SnO₂ is 0.05% to 1%.

(Constitution 3)

The glass substrate for the information recording medium according toconstitution 1 or 2, including, as expressed in terms of percent by masson the oxide basis:

0 to 20% of MgO, and/or

0 to 10% of CaO, and/or

0 to 10% of ZnO, and/or

0 to 13% of ZrO₂, and/or

0 to 16% of TiO₂, and/or

0 to 6% of B₂O₃, and/or

0 to 10% of Na₂O, and/or

0 to 7% of K₂O.

(Constitution 4)

The glass substrate for the information recording medium according toany of constitutions 1 to 3, wherein the glass substrate does notcontain As₂O₃ component and Sb₂O₃ component on the oxide basis, and Cl⁻,NO⁻, SO₂ ⁻ and F⁻ components.

(Constitution 5)

The glass substrate for the information recording medium according toany of constitutions 1 to 3, wherein the glass substrate does notcontain BaO component or SrO component on the oxide basis.

(Constitution 6)

The glass substrate for the information recording medium according toany of constitutions 1 to 5,

wherein, when the content ratio of Li₂O component on the oxide basis inthe region inside the glass substrate for the information recordingmedium which extends from an end surface to a depth of 5 μm toward thecenter is designated as α%, and the content ratio of the Li₂O componenton the oxide basis in the region in the glass substrate for theinformation recording medium which extends from beyond a depth of 5 μmfrom an end surface toward the center, within the region inside theglass substrate which extends from a depth of 5 μm from one main surfaceto a depth of 5 μm from the other main surface along the thicknessdirection, is designated as β%, the ratio of α/β is 1 or less.

(Constitution 7)

The glass substrate for the information recording medium according toany of constitutions 1 to 6, wherein the surface roughness Ra(arithmetic mean roughness) is 2 A (angstrom) or less.

(Constitution 8)

An information recording medium making use of the glass substrate forthe information recording medium according to any of constitutions 1 to7.

(Constitution 9)

A method for producing a glass substrate for an information recordingmedium, including

producing a glass substrate containing, as expressed in terms of percentby mass on the oxide basis:

52 to 67% of SiO₂,

3 to 15% of Al₂O₃, and

0.2. to 8% of P₂O₅;

substituting the alkali ions present at the surface of the glasssubstrate with other ions to produce an ion exchange region; and

eliminating the ion exchange region present at the two main surfaces ofthe glass substrate.

(Constitution 10)

The method for producing a glass substrate for the information recordingmedium according to constitution 9, wherein the substituting is achievedby substituting the alkali component present at the surface layer withan alkali component having a larger ionic radius.

(Constitution 11)

The method for producing a glass substrate for an information recordingmedium according to constitution 10, wherein the glass substrate isheated and then rapidly cooled, thereby forming a compressive stresslayer at the surface.

According to the present invention, a glass substrate can haveproperties that are required in the use as a substrate for aninformation recording medium of the next generation, such as specificgravity, Young's modulus and Vickers hardness. In particular, the glasssubstrate has a good balance between specific gravity and mechanicalstrength on the premise of using the glass substrate in a dynamicenvironment as a substrate for an information recording medium of thenext generation, and has high strength to withstand high speed rotationor drop impact. Furthermore, the glass substrate can acquire highlysmooth surface properties by applying currently established processingtechnologies such as polishing. There can be also provided a glasssubstrate capable of offering more excellent effects than conventionalones, which can be refined even without using arsenic components orantimony components that are feared to cause adverse effects on humanbody and environment, and can also suppress the occurrence of reboil atthe time of direct press molding or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph plotting temperature against viscosity in the cases ofExamples 5, 6, 8 and 44 of the present invention, wherein the verticalaxis represents the logarithm of viscosity (dPa·s), logη, and thehorizontal axis represents the temperature (° C.) of glass indicated bya drawing ball viscometer; and

FIG. 2 is an image (field of vision 3 μm²) obtained by observing with anAFM (atomic force microscope) a polished surface of the substrate forthe information recording medium of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described by way of specific exemplaryembodiments.

The term “glass substrate” of the present invention as used hereingenerically refers to an amorphous glass substrate, and upon statingabout the respective compositional components constituting this glasssubstrate, unless stated otherwise, the contents of the variouscomponents are expressed in terms of mass percent on the oxide basis.Here, the term “on the oxide basis” is directed to a method ofindicating the composition of the respective components contained in theglass under an assumption that the oxides, carbonates and the like usedas raw materials of the constituent components of the glass of thepresent invention are all decomposed upon melting and converted to theindicated oxides, and the amounts of the respective components containedin the glass are indicated on the basis of the total sum of mass ofthese produced oxides as 100 mass %.

The glass used in the glass substrate for the information recordingmedium of the present invention contains a SiO₂ component, an Al₂O₃component and a P₂O₅ component at the proportions of specific rangesshown below.

The present inventors discovered, with regard to the composition rangeof the glass of the present invention, that it is preferable to maintainthe specific gravity of the glass at 2.60 or less, in order to reducethe load exerted on a spindle motor and to obtain a good balance betweenspecific gravity and mechanical strength that can be applied in the useas a substrate for an information recording medium of the nextgeneration. In order to make the balance mentioned above even better, itis most preferable to maintain the specific gravity at 2.595 or less. Onthe other hand, if the specific gravity is lower than 2.20, it issubstantially difficult to obtain a substrate having a desired toughnesswith the glass composition range of the present invention. Thus, it ispreferable to maintain the specific gravity at 2.20 or greater, morepreferably at 2.30 or greater, and most preferably at 2.35 or greater.

Young's modulus will be discussed. As described above, it is under wayto make information recording medium disk substrates capable of highspeed rotation, so as to improve the recording density and data transferrate. However, in order to deal with this trend, the substrate materialmust have high toughness and low specific gravity to prevent diskvibration caused by bending upon high speed rotation. Furthermore, inthe case of contact with a head or in the case of use in portablerecording apparatuses such as removable recording apparatuses, it ispreferable for the glass substrate to have a mechanical strength thatallows the glass substrate to sufficiently withstand the situations, aswell as high Young's modulus and surface hardness. When the Young'smodulus is maintained at 85 GPa or greater in the composition range ofthe glass of the present invention, it is possible to satisfy otherproperties as well. This Young's modulus is more preferably 85.5 GPa orgreater, and most preferably 86 GPa or greater.

However, even though the glass substrate has simply high toughness, ifthe glass substrate has high specific gravity, bending occurs upon highspeed rotation due to the large weight of the glass substrate, and theglass substrate generates vibration. On the contrary, even though theglass substrate has low specific gravity, if the glass substrate hassmall toughness, similarly vibration is generated. Furthermore, there isa problem that an increase in the weight causes an increase in theelectric power consumption. In addition, if the specific gravity is madeexcessively low, consequently it becomes difficult to obtain a desiredmechanical strength. Therefore, a balance must be taken betweenapparently contradictory characteristics such as high toughness and lowspecific gravity, and its preferred range is such that the valueexpressed in Young's modulus [GPa]/specific gravity is 33.0 or greater,more preferably 33.5 or greater, and most preferably 33.9 or greater.Here, since it is better with higher values of specific modulus, theupper limit value is not particularly limited.

The SiO₂ component is a component that forms the glass network structureand is essentially contained to achieve enhancement of chemicalstability or reduction of specific gravity. When the amount is less than52%, the resulting glass has insufficient chemical durability. Since thespecific gravity tends to increase along with an increase in thecontents of other components, the lower limit of the content of the SiO₂component is preferably 52%, more preferably 53%, and even morepreferably 54%. Furthermore, if the content is greater than 67%, anincrease in viscosity leads to difficulties in fusion and press molding,and homogeneity of the material or refining effects are prone to bereduced. Accordingly, it is preferable to set the upper limit of thecontent at 67%, more preferably at 66%, and most preferably at 65%.

The Al₂O₃ component is an important component that contributes tostabilization of the glass and enhancement of chemical durability.However, if the amount is less than 3%, the effects are insufficiently.Therefore, the lower limit of the content is preferably 3%, morepreferably 4%, and most preferably 5%. Furthermore, if the amount isgreater than 15%, fusibility, moldability and resistance todevitrification are rather deteriorated, and homogeneity or refiningeffects are prone to be reduced. Accordingly, it is preferable to setthe upper limit of the content at 15%, more preferably at 14%, and mostpreferably at 13%.

The P₂O₅ component offers an effect of suppressing crack propagation inthe glass, and therefore can contribute to an increase in Vickershardness. On the other hand, the component contributes to viscosityreduction, and also enhances melting and ref inability of raw glass whenpresent together with SiO₂. In order to obtain these effects, it isnecessary to set the lower limit of the content at least 0.2%. In orderto obtain these effects more easily, it is preferable to set the lowerlimit of the content at 0.3%, and most preferably at 0.5%. However, ifthis component is added in excess, it becomes to achieve vitrification,and devitrification or phase separation is prone to occur. Thus, it ispreferable to set the upper limit of the content at 8%, more preferablyat 6%, and most preferably at 4%.

The Li₂O component is a component which is preferably added so as topromote the reduction of viscosity of the glass, enhancement ofmoldability, enhancement of homogeneity, and chemical strengthening. Onthe other hand, chemical durability is important in the use of the glassas a substrate for an information recording medium, the amount of elutedalkali components should be suppressed to the minimum. Therefore, thelower limit of the content of the Li₂O component is preferably 2.9%, andmore preferably 3.0%. The upper limit of the content of the Li₂Ocomponent is preferably set at 8%, more preferably at 7.5%, and mostpreferably at 7%.

The respective components of TiO₂, ZrO₂, MgO, CaO and ZnO are componentsthat contribute to enhancement of Young's modulus, while generallysuppressing the increase of the specific gravity of the glass as much aspossible. The present inventors found that when the total amount ofthese components is adjusted to a specific range, it is easier to obtaina desired value of the ratio of Young's modulus to specific gravity.That is, if the total amount of one or more selected from thesecomponents exceeds 20%, the specific gravity value becomes excessivelylarge. If the total amount thereof is less than 10%, the Young's modulusand the ratio of Young's modulus to specific gravity cannot besatisfied. Therefore, the upper limit of the total content of thesecomponents is preferably 20%, and more preferably 19%, and an even morepreferred upper limit value is 18%. Furthermore, the lower limit valueis preferably 5%, more preferably 11%, and even more preferably 12%.

The MgO, CaO and ZnO components are components that contribute to thereduction of specific gravity of the glass and enhancement of Young'smodulus, and since they are also effective in the reduction of viscosityof the glass, they can be added as optional components. However, if thecontent of MgO exceeds 20%, the content of CaO exceeds 10%, or thecontent of ZnO exceeds 10%, the specific gravity of raw glass isincreased, and it is difficult to obtain a desired glass. Therefore, theupper limits of the contents of these components are 20% for MgO, 10%for CaO and 10% for ZnO, more preferred upper limit values are 18% forMgO, 8% for CaO and 7% for ZnO, and even more preferred upper limitvalues are 15% for MgO, 6% for CaO and 5% for ZnO.

The ZrO₂ component contributes to enhancement of Young's modulus of theglass and enhancement of chemical durability, and thus can be optionallyadded. However, if the amount of addition of this component exceeds 11%,a melt residue or ZrSiO₄ (zircon) is likely to be generated, and theglass specific gravity is increased. Therefore, it is preferable to setthe upper limit of the content at 13%, more preferably at 10%, and mostpreferably at 9%.

The TiO₂ component is a component that contributes to enhancement ofYoung's modulus of the glass, viscosity reduction and enhancement ofchemical durability, and can be optionally added. However, if the amountof addition of this component exceeds 16%, the specific gravity of theglass is increased, and furthermore, vitrification is difficult toachieve. Therefore, the upper limit of the content is preferably set at16%, more preferably at 13%, and most preferably at 12%.

The B₂O₃ component contributes to the reduction of viscosity of theglass and improves solubility and moldability, and thus the componentcan be added as an optional component. However, if the content of thiscomponent exceeds 6%, it is difficult to satisfy the mechanicalproperties, while raw glass easily undergoes phase separation, andvitrification is difficult to achieve. Therefore, it is preferable toset the upper limit of the content at 6%. Amore preferred upper limitvalue is 5.5%.

The Na₂O component and K₂O component are components that bring aboutreduction of viscosity of the glass, enhancement of moldability andenhancement of homogeneity, similarly to the case of the Li₂O component,but these components make the specific gravity of the glass higher ascompared with the Li₂O component. However, since these components havelarger ionic radii than that of the Li₂O component, their effect ofalkali migration from the substrate is smaller compared with Li, andsince these components give an effect that makes it easier to design theglass to have a higher average coefficient of linear expansion, thesecomponents may be incorporated together with the Li₂O component. Evenunder such circumstances, the amount of eluted alkali components must bemaintained at the minimum required. Therefore, the upper limit of thecontent of the Na₂O component is preferably 10%, more preferably 9%, andmost preferably 8%. The upper limit of the content of the K₂O componentis preferably set at 7%, more preferably at 5%, and most preferably at3%.

The BaO component and the SrO component are components that areeffective in the reduction of viscosity of the glass, enhancement ofchemical durability and mechanical improvement. These components havethe same function as that of the MgO, CaO and ZnO component, but on theother hand, they tend to render the glass specific gravity higher thanthe MgO, CaO and ZnO components. Therefore, it is preferable not toincorporate these components as far as possible.

In order to obtain high refining effects while maintaining theproperties required of a substrate for an information recording medium,it is preferable to incorporate one or more components selected from aSnO₂ component and a CeO₂ component, as a main refining component. Inorder to obtain a high refining effect, the lower limit of the contentof the SnO₂ component or the CeO₂ component, or of the total content ofthe two components, is preferably 0.05%, more preferably 0.1%, and mostpreferably 0.15% on the oxide basis.

On the other hand, in order to maintain the specific gravity low andobtain a high refining effect while maintaining the mechanical strength,and to increase the reboil suppressing effect upon direct pressing, theupper limit of the content of one or more components selected from theSnO₂ component and the CeO₂ component is preferably 1%, more preferably0.7%, and most preferably 0.5%.

An As₂O₃ component or a Sb₂O₃ component, and Cl⁻, NO⁻, SO₂ ⁻ and F⁻components operate as refining agents, but they are components which maybe harmful to the environment, and use thereof should be avoided. Theglass of the present invention can have a refining effect even withoutcontaining the As₂O₃ component or the Sb₂O₃ component, and when thesecomponents and the refining agent component of the present invention areadded together, the refining agents may cancel out the refining effectsof each other.

Gd₂O₃, La₂O₃, Y₂O₃, Nb₂O₅, Ga₂O₃ and WO₃ components contribute to thereduction of viscosity of the glass, an enhancement of mechanicalproperties due to enhancement of Young's modulus, and enhancement ofheat resistance, and can be therefore added as optional components.However, an increase in the amount of addition calls for an increase inthe specific gravity or an increase in the cost for raw materials.Therefore, the amount is such that the total amount of one or more ofthese components is sufficient at up to 5%, and if the total amountexceeds 5%, the specific gravity, Young's modulus and specific modulusof rigidity are not satisfied. Therefore, it is preferable to set theupper limit of the total amount of these components at 5%, morepreferably at 4%, and most preferably at 3%.

V, Cu, Mn, Cr, Co, Mo, Ni, Fe, Te, Pr, Nd, Er, Eu, Sm components and thelike, which are used as colorant components for glass, can be added forthe purpose of distinguishing the type of glass by using the fluorescentproperty caused by these components and preventing mixing with othertypes of glass at the site of production or the like. However, sincethese components bring about an increase in specific gravity, anincrease in the cost for raw materials and a decrease in glass formingcapability, the amount is such that the total amount of one or more ofthese components is sufficient at up to 5%. Therefore, the upper limitof the total amount of these components is preferably set at 5%, morepreferably at 4%, and most preferably at 3%, on the oxide basis.

In the glass substrate of the present invention, the damage caused byalkali migration to the magnetic film formed on the substrate surfacecan be suppressed, as the content of Li present at the surface isdecreased by substituting Li with other components having larger ionicradii than that of Li.

In particular, many of the causes of alkali migration are caused byelution of alkali components from an end surface of the substrate wherea magnetic film or the like is not formed.

For this reason, it is preferable to reduce the Li content particularlyat near the substrate end surfaces, and it is preferable that, when thecontent ratio of Li₂O component on the oxide basis in the region in theglass substrate for an information recording medium which extends froman end surface to a depth of 5 μm toward the center (hereinafter,referred to as “end surface region”) is designated as α%, and thecontent ratio of the Li₂O component on the oxide basis in the region inthe glass substrate for the information recording medium which extendsfrom beyond a depth of 5 μm from an end surface toward the center(hereinafter, referred to as “inner region”), within the region insidethe glass substrate which extends from a depth of 5 μm from one mainsurface to a depth of 5 μm from the other main surface along thethickness direction, is designated as β%, the ratio α/β be such thatα/β≦1.

In the actual measurement of the Li content, it is advantageous tomainly use, for example, a method according to the ICP-AES method bysampling a portion of the glass in the end surface region and the innerregion.

In the actual production process, the content ratio of the Li₂Ocomponent can be adjusted to the value described above, by substitutingLi with another component, for example, Na or K, through an ion exchangetreatment of the substrate obtained after polish processing. In thiscase, the two main surfaces of the glass substrate may be in a state ofcontaining a reduced proportion of the Li₂O component as compared withthe inner region by ion exchange, but upon considering the specificgravity of the substrate as a whole, it is preferable that the contentratio of the Li₂O component at the two main surfaces be equivalent tothat in the inner region.

In order to achieve this state, a substrate may be produced bysubjecting a blank to surface processing, subsequently substituting Liwith another component, for example, Na or K, through an ion exchangetreatment, and then eliminating the two main surfaces by polishprocessing.

Furthermore, if the glass substrate of the present invention is providedwith a compressive stress layer on the surface, an effect of furtherenhancing the mechanical strength can be obtained.

As a method of forming a compressive stress layer, for example, there isavailable a chemical strengthening method involving an exchange reactionof the alkali component present at the surface layer of the glasssubstrate before forming a compressive stress layer, with an alkalicomponent having a larger ionic radius. There are also available athermal strengthening method of heating a glass substrate and thenrapidly cooling the glass substrate; and an ion injection method ofinjecting ions to the surface layer of the glass substrate.

In the chemical strengthening method, for example, glass is immersed inthe molten salt of a salt containing potassium or sodium, for example,potassium nitrate (KNO₃), sodium nitrate (NaNO₃) or a composite saltthereof, at a temperature of 300 to 600° C. for 0.5 to 12 hours.Thereby, an exchange reaction of the lithium component (Li⁺ ion) presentin the glass components at near the substrate surface, with a sodiumcomponent (Na⁺ ion) or a potassium component (K⁺ ion), which are bothalkali components with larger ionic radii than that of Li; or anexchange reaction of the sodium component (Na⁺ ion) present in the glasscomponents at near the substrate surface, with a potassium componentwhich is an alkali component having a larger ionic radium than that ofthe sodium component, proceeds. Then, there occurs an increase in volumeof crystallized glass, and a compressive stress is generated inside theglass substrate surface layer. As a result, the ring flexural strength,which is an index of impact properties, undergoes an increase.

The thermal strengthening method is not particularly limited, but forexample, a compressive stress layer which is generated by thetemperature difference between the surface and the inside of the glasssubstrate, can be formed by heating the glass substrate to 300° C. to600° C., and then conducting rapid cooling such as water cooling and/orair cooling. Furthermore, when the thermal strengthening method iscombined with the chemical strengthening method described above, acompressive stress layer can be formed more effectively.

In regard to the glass substrate of the present invention, themechanical strength or Young's modulus can be increased by inducingphase separation or generating crystals inside the glass by heattreating a bulk glass body at room temperature, which has been obtainedby melting. Upon producing crystallized glass, a glass raw materialcontaining the raw material of the various components mentioned above ismelted and rapidly cooled to thereby produce raw glass, and the rawglass is heat treated and subjected to a nucleation process. After thisnucleation process, the raw glass is heat treated at a temperaturehigher than that used in the nucleation process, and thereby a crystalgrowth process is carried out.

In order to produce a substrate for an information recording medium ofthe present invention, molten glass produced under the conditionsdescribed above is dropped into a lower frame, and the glass is pressedwith an upper frame and a lower frame, thereby molding the glass into adisk shape. The glass substrate is subjected to shaping processingaccording to necessity, and then may be subjected to lapping processingand polish processing by known methods. According to the presentinvention, when currently established processing methods such aspolishing are used, surface properties with an Ra of 2 A (angstrom) orless can be obtained. Furthermore, also when the glass substrate iswashed with, for example, an acid such as hydrofluoric acid, or analkali so as to eliminate residual polishing materials during the polishprocessing, surface properties with an Ra of 2 A (angstrom) or less canbe maintained.

More specifically, the glass substrate for the information recordingmedium of the present invention is produced by the following method.First, raw materials such as oxides, carbonates and nitrates are mixedso as to have the glass constituent components in the composition rangesdescribed above, and the mixture is melted using a conventional meltingapparatus which makes use of a crucible made of platinum, quartz or thelike, to a temperature which gives a viscosity of the glass melt of 1.5to 3.0 dPa·s. Next, the temperature of the glass melt is raised to atemperature which gives a viscosity of 1.0 to 2.3 dPa·s, and preferably1.2 to 2.2 dPa·s, and bubbles are generated within the glass melt toinduce a stirring effect, so that the degree of homogeneity isincreased. Subsequently, the temperature of the glass melt is lowered toa temperature which gives a viscosity of 1.8 to 2.6 dPa·s, andpreferably 2.0 to 2.5 dPa·s, and removal of the bubbles generated insidethe glass and refining are carried out. Then, this temperature ismaintained.

Next, the temperature of the upper frame of the press molding frames isset at 300±100° C., and preferably at 300±50° C., and the temperature ofthe lower frame is set at Tg of the glass ±50° C., and preferably Tg±30°C.

Furthermore, the temperature of a glass discharge pipe for guiding theglass from the crucible to the press molding frames, is set at atemperature which gives a viscosity of the glass of 2.0 to 2.6 dPa·s,and preferably 2.1 to 2.5 dPa·s. A predetermined amount of the glass isdropped on the lower frame, the upper frame and the lower frame areapproached to press, and thereby a glass molded product is obtained.

In the production of the substrate for the information recording medium,since cost reduction per sheet of glass is demanded, pressing is carriedout at high speed, such as a press speed of 150 to 700 mm/sec and acycle time (time taken from the initiation of pressing to a subsequentinitiation of pressing) of 1 to 2 seconds. However, even under theimpact of such pressing process, when the glass of the present inventionis used, and the temperature of the glass melt and the temperature ofthe production apparatus are managed as described above, the occurrenceof reboil during pressing can be suppressed.

Next, the glass substrate is lapped with abrasive grains having anaverage particle size of 5 to 30 μm, for about 10 minutes to 60 minutes,and after inner and outer diameter processing, the glass substrate ispolished for about 30 minutes to 60 minutes using loose abrasive grainsmade of cerium oxide or the like, which have an average particle sizeobtained of 0.5 μm to 2 μm. Thereby, a substrate for an informationrecording medium can be obtained. The lapping and polishing processesare not limited to the processes as described above, and known methodsmay be appropriately used.

EXAMPLES

Hereinafter, suitable Examples of the present invention will beexplained. However, the present invention is not intended to be limitedto these Examples.

The glasses of the Examples of the present invention were all producedas follows. Raw materials of oxides and carbonates were mixed, and thismixture was fused at a temperature of about 1200 to 1400° C. using acrucible made of quartz or platinum. The batch that served as the rawmaterial was sufficiently fused so that any melt residue would not begenerated. The resulting batch was heated to a temperature of about 1350to 1500° C. and then cooled to a temperature of 1450 to 1250° C., so asto carry out removal of the bubbles generated inside the glass andrefining. Subsequently, while the temperature was maintained, apredetermined amount of glass was discharged and molded into a diskshape by a direct press method, with the temperature of the upper frameset at 300±100° C. and the temperature of the lower frame set at Tg±50°C. The molded disk was cooled, and thus a glass molded product wasobtained. Subsequently, the obtained glass molded product was subjectedto lapping and polishing by the method described above, and to washingwith hydrofluoric acid for removal of abrasives, and thus a substratefor an information recording medium was obtained. The surface roughnessRa (arithmetic mean roughness) of the substrates thus obtained was all 2A (angstrom) or less. Here, the surface roughness Ra (arithmetic meanroughness) was measured with an atomic force microscope (AFM).

The glass composition (mass %), specific gravity of the substrateobtained after press molding, Vickers hardness, Young's modulus, theratio of Young's modulus to specific gravity (Young's modulus/specificgravity), and the average coefficient of linear expansion at 25° C. to100° C., of Examples 1 to 44 and Comparative Examples 1 and 2 arepresented in Table 1 to Table 8.

Furthermore, a temperature-viscosity graph of the glass of the presentinvention is shown in FIG. 1.

The viscosity of the glass was measured using a drawing ball viscometer(BVM-13LH manufactured by OPT Corp.).

The average coefficient of linear expansion refers to a value measuredin conformity with JOGIS (Japanese Optical Glass Industrial Standard)16-2003 “Measuring method for average linear thermal expansioncoefficient of optical glass at normal temperature,” by varying thetemperature range from 25° C. to 100° C.

The specific gravity was measured using an Archimedes's method, whileYoung's modulus was measured using an ultrasonic method.

Vickers hardness refers to a value obtained by measuring a load (N)using a square pyramid-shaped diamond indenter with opposite sidesmeeting at the apex at an angle of 136°, by making a pyramid-shapedindentation on the test surface, and dividing the load by the surfacearea (mm²) calculated from the length of the indentation. Themeasurement was made using a microhardness meter MVK-E manufactured byAkashi Seishakusho, Ltd., under a test load of 2.94 (N) and at aretention time of 15 (seconds).

TABLE 1 Example No. 1 2 3 4 5 6 SiO₂ 59.60 59.60 59.60 59.60 60.10 61.60P₂O₅ 3.00 3.00 3.00 3.00 3.00 3.00 Al₂O₃ 8.00 8.00 8.00 8.00 8.00 9.00B₂O₃ 2.00 2.00 2.00 1.00 Li₂O 6.00 6.00 6.00 6.00 5.00 5.00 K₂O 2.002.00 2.00 2.00 1.00 1.00 Na₂O 4.00 4.00 6.00 4.00 8.00 6.00 MgO 1.503.50 1.50 1.50 3.50 1.50 CaO 1.00 1.00 1.00 1.00 1.00 ZnO 1.50 1.50 1.501.50 1.50 ZrO₂ 4.00 2.00 2.00 3.00 8.00 6.00 TiO₂ 7.00 9.00 9.00 8.001.00 3.00 SnO₂ 0.40 0.40 CeO₂ 0.40 0.40 0.40 0.40 Total 100 100 100 100100 100 MgO + CaO + ZnO + TiO₂ + ZrO₂ 15.00 17.00 15.00 15.00 12.5013.00 Specific gravity 2.561 2.576 2.571 2.557 2.561 2.534 Vickershardness 700 720 700 705 700 690 Young's modulus (GPa) 89.3 89.6 88.989.0 87.9 86.1 Young's 34.9 34.8 34.6 34.8 34.3 34.0 modulus/specificgravity Average coefficient of 73 73 76 73 75 68 linear expansion (×10⁻⁷· ° C.⁻¹)

TABLE 2 Example No. 7 8 9 10 11 12 SiO₂ 60.60 61.60 64.60 63.60 59.6059.60 P₂O₅ 3.00 3.00 2.00 2.00 3.00 3.00 Al₂O₃ 9.00 8.00 6.00 6.00 8.008.00 B₂O₃ 3.00 1.00 1.00 1.00 2.00 2.00 Li₂O 5.00 5.00 4.00 4.00 5.005.00 K₂O 1.00 1.00 1.00 1.00 Na₂O 6.00 6.00 6.00 5.00 6.00 6.00 MgO 2.002.00 3.00 3.00 3.00 3.00 CaO 2.00 1.00 1.00 1.00 ZnO ZrO₂ 5.00 4.00 4.004.00 4.00 3.00 TiO₂ 3.00 7.00 9.00 11.00 7.00 8.00 SnO_(□) 0.20 CeO₂0.40 0.40 0.20 0.40 0.40 0.40 Total 100 100 100 100 100 100 MgO + CaO +ZnO + TiO₂ + ZrO₂ 12.00 14.00 16.00 18.00 15.00 15.00 Specific gravity2.532 2.542 2.540 2.551 2.556 2.551 Vickers hardness 700 720 730 720 700700 Young's modulus (GPa) 86.1 86.5 86.8 87.9 87.9 88.1 Young's 34.034.0 34.2 34.5 34.4 34.5 modulus/specific gravity Average coefficient of71 70 68 69 70 71 linear expansion (×10⁻⁷ · ° C.⁻¹)

TABLE 3 Example No. 13 14 15 16 17 18 SiO₂ 60.60 59.60 61.10 60.60 59.6058.60 P₂O₅ 2.00 2.00 3.00 3.00 2.00 2.00 Al₂O₃ 8.00 8.00 8.00 8.00 8.008.00 B₂O₃ 2.00 2.00 1.00 1.00 2.00 2.00 Li₂O 5.00 5.00 5.00 5.00 4.004.00 K₂O 1.00 1.00 1.00 1.00 1.00 1.00 Na₂O 6.00 6.00 6.00 6.00 7.007.00 MgO 3.00 3.00 2.00 2.00 3.00 3.00 CaO 1.00 1.00 1.00 1.00 1.00 1.00ZnO ZrO₂ 3.00 3.00 4.00 4.00 3.00 3.00 TiO₂ 8.00 9.00 7.00 7.00 9.009.00 WO₃ 0.50 1.00 1.00 CeO₂ 0.40 0.40 0.40 0.40 0.40 0.40 Total 100 100100 100 100 100 MgO + CaO + ZnO + TiO₂ + ZrO₂ 15.00 16.00 14.00 14.0016.00 16.00 Specific gravity 2.556 2.567 2.552 2.560 2.568 2.585 Vickershardness 700 700 710 700 700 700 Young's modulus (GPa) 88.9 88.2 87.288.1 87.4 88.0 Young's 34.8 34.3 34.2 34.4 34.0 34.0 modulus/specificgravity Average coefficient of 70 70 69 70 70 70 linear expansion (×10⁻⁷· ° C.⁻¹)

TABLE 4 Example No. 19 20 21 22 23 24 SiO₂ 59.60 57.60 58.60 58.60 57.6057.60 P₂O₅ 3.00 3.00 3.00 3.00 3.00 3.00 Al₂O₃ 10.00 12.00 8.00 8.008.00 8.00 B₂O₃ 1.00 1.00 1.00 1.00 2.00 2.00 Li₂O 5.00 5.00 5.00 5.005.00 5.00 K₂O 1.00 1.00 2.00 2.00 2.00 2.00 Na₂O 6.00 6.00 8.00 8.008.00 8.00 MgO 2.00 2.00 2.00 2.00 2.00 2.00 CaO 1.00 1.00 1.00 1.00 1.001.00 ZnO ZrO₂ 4.00 4.00 4.00 3.00 3.00 2.00 TiO₂ 7.00 7.00 7.00 8.008.00 9.00 WO₃ CeO₂ 0.40 0.40 0.40 0.40 0.40 0.40 Total 100 100 100 100100 100 MgO + CaO + ZnO + TiO₂ + ZrO₂ 14.00 14.00 14.00 14.00 14.0014.00 Specific gravity 2.546 2.552 2.571 2.565 2.567 2.564 Vickershardness 690 690 700 700 700 700 Young's modulus (GPa) 87.5 88.3 87.687.1 87.7 87.5 Young's 34.4 34.6 34.1 34.0 34.2 34.1 modulus/specificgravity Average coefficient of 70 71 72 73 72 71 linear expansion (×10⁻⁷· ° C.⁻¹)

TABLE 5 Example No. 25 26 27 28 29 30 SiO₂ 58.60 58.60 59.60 58.60 57.6058.10 P₂O₅ 3.00 3.00 1.00 1.00 1.00 1.00 Al₂O₃ 8.00 8.00 8.00 8.00 8.008.00 B₂O₃ 2.00 3.00 3.00 3.00 5.00 3.00 Li₂O 5.00 5.00 5.00 5.00 5.005.00 K₂O 2.00 2.00 2.00 2.00 2.00 2.00 Na₂O 6.00 5.00 6.00 6.00 6.006.00 MgO 1.50 1.50 3.00 3.00 3.00 4.50 CaO 1.00 1.00 1.00 1.00 1.00 1.00ZnO 1.50 1.50 ZrO₂ 4.00 4.00 2.00 2.00 4.00 4.00 TiO₂ 7.00 7.00 9.007.00 7.00 7.00 WO₃ 3.00 CeO₂ 0.40 0.40 0.40 0.40 0.40 0.40 Total 100 100100 100 100 100 MgO + CaO + ZnO + TiO₂ + ZrO₂ 15.00 15.00 15.00 13.0015.00 16.50 Specific gravity 2.572 2.561 2.561 2.590 2.569 2.586 Vickershardness 710 710 700 710 700 700 Young's modulus (GPa) 90.8 89.3 89.488.0 89.0 90.8 Young's 35.3 34.9 34.9 34.0 34.6 35.1 modulus/specificgravity Average coefficient of 71 70 72 71 70 71 linear expansion (×10⁻⁷· ° C.⁻¹)

TABLE 6 Example No. 31 32 33 34 35 36 SiO₂ 58.10 58.10 58.10 58.10 58.1058.10 P₂O₅ 1.00 1.00 1.00 1.00 1.00 1.00 Al₂O₃ 8.00 8.00 8.00 8.00 8.008.00 B₂O₃ 3.00 3.00 3.00 3.00 3.00 3.00 Li₂O 5.00 5.00 5.00 5.00 5.005.00 K₂O 2.00 2.00 2.00 2.00 2.00 2.00 Na₂O 6.00 6.00 5.00 6.00 6.006.00 MgO 3.00 3.00 6.00 5.00 2.50 7.50 CaO 2.50 2.50 2.50 2.50 5.00 ZnOZrO₂ 4.00 2.00 2.00 2.00 2.00 2.00 TiO₂ 7.00 9.00 7.00 7.00 7.00 7.00WO₃ CeO₂ 0.40 0.40 0.40 0.40 0.40 0.40 Total 100 100 100 100 100 100MgO + CaO + ZnO + TiO₂ + ZrO₂ 16.50 16.50 17.50 16.50 16.50 16.50Specific gravity 2.593 2.583 2.580 2.583 2.588 2.568 Vickers hardness710 710 710 700 700 700 Young's modulus (GPa) 89.5 90.2 90.2 89.9 89.789.5 Young's 34.5 34.9 35.0 34.8 34.7 34.9 modulus/specific gravityAverage coefficient of 71 71 74 75 75 78 linear expansion (×10⁻⁷ · °C.⁻¹)

TABLE 7 Example No. 37 38 39 40 41 42 SiO₂ 58.10 58.10 58.10 58.10 59.6059.60 P₂O₅ 1.00 1.00 1.00 1.00 1.00 1.00 Al₂O₃ 8.00 8.00 8.00 8.00 8.008.00 B₂O₃ 3.00 3.00 3.00 3.00 3.00 3.00 Li₂O 5.00 5.00 5.00 5.00 5.005.00 K₂O 2.00 2.00 2.00 2.00 2.00 2.00 Na₂O 6.00 6.00 6.00 6.00 4.004.00 MgO 7.50 6.00 6.50 7.50 8.00 13.00 CaO 5.00 2.50 4.00 2.00 5.00 ZnOZrO₂ 2.00 1.00 2.00 2.00 2.00 2.00 TiO₂ 2.00 7.00 4.00 5.00 2.00 2.00WO₃ CeO₂ 0.40 0.40 0.40 0.40 0.40 0.40 Total 100 100 100 100 100 100MgO + CaO + ZnO + TiO₂ + ZrO₂ 16.50 16.50 16.50 16.50 17.00 17.00Specific gravity 2.580 2.573 2.579 2.575 2.572 2.555 Vickers hardness700 710 710 710 720 720 Young's modulus (GPa) 91.3 90.2 90.3 90.4 91.491.3 Young's 35.4 35.1 35.0 35.1 35.6 35.7 modulus/specific gravityAverage coefficient of 79 80 79 79 69 71 linear expansion (×10⁻⁷ · °C.⁻¹)

TABLE 8 Example No. Comparative Comparative 43 44 Example 1 Example 2SiO₂ 58.10 58.10 60.8 67.0 P₂O₅ 1.00 1.00 Al₂O₃ 9.50 9.50 4.0 11.8 B₂O₃3.00 3.00 2.5 Li₂O 5.00 5.00 7.3 7.0 K₂O 2.00 2.00 2.0 Na₂O 4.00 4.003.9 3.5 MgO 8.00 13.00 4.3 CaO 5.00 5.8 ZnO ZrO₂ 2.00 2.00 6.4 2.0 TiO₂2.00 2.00 7.3 2.0 WO₃ La₂O₃ 2.0 CeO₂ 0.40 0.40 Sb₂O₃ Sb₂O₃ 0.2 0.2 Total100 100 100 100 MgO + CaO + ZnO + 17.00 17.00 23.8 4.0 TiO₂ + ZrO₂Specific gravity 2.574 2.562 2.685 2.48 Vickers hardness 710 710 640 610Young's modulus (GPa) 91.2 92.2 98.6 83 Young's 35.4 36.0 36.7 33.5modulus/specific gravity Average coefficient of 70 69 70 66 linearexpansion (×10⁻⁷ · ° C.⁻¹)

Example 45

A 2.5-inch polished substrate for HDD (65φ×0.635 mmt) of Example 8 wasimmersed in a salt mixture of potassium nitrate and sodium nitrate(KNO₃:NaNO₃=1:3) at 400° C. for 0.25 hours, and thereby a compressivestress layer was formed at the surface. It was confirmed that thissubstrate had its ring flexural strength improved to a value four timeslarger than the value obtained before forming the compressive stresslayer (260 MPa). Here, the ring flexural strength means a flexuralstrength measured by a concentric ring bending method, in which a thindisk-shaped specimen having a diameter of 65 mm and a thickness of 0.635mm is produced, and the strength of the disk-shaped specimen is measuredusing circular-shaped support ring and load ring.

Example 46

A 2.5-inch polished substrate for HDD (65φ×0.635 mmt) of Example 44 wasimmersed in a salt mixture of potassium nitrate and sodium nitrate(KNO₃:NaNO₃=1:3) at 400° C. for 0.5 hours, and thereby a compressivestress layer was formed at the surface. It was confirmed that thissubstrate had its ring flexural strength improved to a value eight timeslarger than the value obtained before forming the compressive stresslayer (270 MPa).

Example 47

A 2.5-inch polished substrate for HDD (65φ×0.635 mmt) of Example 8 washeated to 300° C. to 600° C. and then was subjected to rapid cooling byan air cooling method, and thereby a compressive stress layer was formedat the surface. It was confirmed that this substrate had its ringflexural strength improved.

Example 48

A 2.5-inch polished substrate for HDD (65φ×0.635 mmt) of Example 34 wasproduced by a known method and was immersed in a salt mixture ofpotassium nitrate and sodium nitrate (KNO₃:NaNO₃=1:3) at 400° C. for0.25 hours, and thereby a compressive stress layer was formed at thesurface. It was confirmed that this substrate had its ring flexuralstrength improved to a value 3 to 6 times larger than the value obtainedbefore forming the compressive stress layer (250 MPa).

Example 49

A 2.5-inch polished substrate for HDD (65φ×0.635 mmt) having the glasscomposition of Example 44 was produced by a known method, throughpolishing processes including a chemical strengthening treatmentprocess, and the substrate surface was observed with an AFM at a fieldof vision of 3 μm². It was confirmed that the substrate had an Ra of 1.4A, an Rq of 1.8 A, an Rmax of 15.5 A, and a microwaviness (μWa) of 0.72A, and that the substrate was very excellent in the surface propertiesrequired of substrates for HDD of next generation.

The microwaviness (μWa) is one of the factors exerting influence on theelectromagnetic conversion properties of magnetic recording media, andin order to obtain excellent electromagnetic conversion properties, themicrowaviness needs to be reduced, as in the case of Ra. Measurement ofthe microwaviness may be carried out by optical interferometry (name ofapparatus: Micro XAM) under the conditions of a band pass filter of 50to 200 nm, along the circumferential direction in the directions of 0°,90°, 180° and 270° of the upper and lower surfaces of the substrate.

Example 50

Furthermore, a chromium alloy undercoat layer and a cobalt alloymagnetic layer were formed on the substrate obtained by the Exampledescribed above, by a DC sputtering method, and a diamond-like carbonlayer was further formed thereon. Subsequently, aperfluoropolyether-based lubricant was applied thereon, and thus amagnetic information recording medium was obtained.

The substrate of the present invention such as a substrate for magneticrecording medium or the like, can be made to have a large surfacerecording density, so that even if the substrate itself is made torotate at high speed in order to increase the recording density, bendingor deformation does not occur, and the vibration due to this rotation isreduced to thereby decrease the track mis-registration (TMR) caused byvibration or bending. Furthermore, since the substrate has excellentimpact resistance properties, it hardly undergoes head crash ordestruction of substrate particularly as an information recording mediumfor mobile applications and the like, and as a result, the substrateexhibits excellent stable operability.

1. A glass substrate for an information recording medium, comprising, asexpressed in terms of percent by mass on an oxide basis: 52 to 67% ofSiO₂, 3 to 14% of Al₂O₃, 0.2 to 8% of P₂O₅, 1.5 to 20% of MgO, 0 to 10%of CaO, 0 to 10% of ZnO, 0 to 13% of ZrO₂, 0 to 16% of TiO₂, 0 to 1% ofCeO₂, 0 to 1% of SnO₂, 0 to 6% of B₂O₃, 4 to 10% of Na₂O, and 0 to 7% ofK₂O, wherein a total amount of MgO, CaO, ZnO, ZrO₂ and TiO₂ in the glasssubstrate is 5 to 20% by mass on the oxide basis, and the total amountof CeO₂ and/or SnO₂ is 0.05 to 1% by mass on the oxide basis, and thetotal amount B₂O₃, Na₂O, and/or K₂O is 4 to 11%) by mass on the oxidebasis, the glass substrate having a Young's modulus of 85 GPa orgreater, a specific gravity of 2.60 or less, and a ratio of Young'smodulus to specific gravity (Young's modulus/specific gravity) of 33.0GPa or greater.
 2. The glass substrate for an information recordingmedium according to claim 1, further comprising, as expressed in termsof percent by mass on the oxide basis: 2.9 to 8% Li₂O.
 3. The glasssubstrate for an information recording medium according to claim 1,wherein the glass substrate does not contain As₂O₃ component and Sb₂O₃component on the oxide basis, and Cl⁻, NO⁻, SO² ⁻ and F⁻ components. 4.The glass substrate for an information recording medium according toclaim 1, wherein the glass substrate does not contain BaO component orSrO component on the oxide basis.
 5. The glass substrate for aninformation recording medium according to claim 1, wherein, when acontent ratio of Li₂O component on the oxide basis in a region insidethe glass substrate for the information recording medium which extendsfrom an end surface to a depth of 5 μm toward the center is designatedas an α%, and the content ratio of the Li₂O component n the oxide basisin the region in the glass substrate for the information recordingmedium which extends from beyond a depth of 5 μm from an end surfacetoward the center, within the region inside the glass substrate whichextends from a depth of 5 μm from one main surface to a depth of 5 μmfrom an other main surface along the thickness direction, is designatedas a β%, the ratio of α/β is 1 or less.
 6. The glass substrate for aninformation recording medium according to claim 1, wherein the surfaceroughness Ra (arithmetic mean roughness) is 2 A or less.
 7. Aninformation recording medium making use of the glass substrate for aninformation recording medium according to claim 1.